It does sound like a fuel restriction of some kind - debris in tank, blocked filter etc.
6 US gall/hr would give no more than about 45% power for a standard parallel valve 8.5 compression IO-360, which at 2200 rpm would normally be at about 19-20" manifold pressure. Full throttle and rich mixture should be around twice that flow rate.
Did the EGT indicate a leaner than normal mixture?
Did you try returning the mixture to fully rich when it happened?
The loss of power given the RPM and fuel flow was probably a lean-cut. I don't know why the alternate air caused a further power loss - possibly it affected the manifold pressure at a given throttle opening.
I would have thought a fuel flow check would give an indication if this was the case.